LONDON — British security sources tell NBC News that two of the five suspects in custody in connection with three recent terror incidents in Scotland and London are medical doctors and one may have assembled the bombs. Authorities also said they believe that most, if not all, of the suspects come from Middle Eastern countries, including one from Iraq.

On Sunday, British police appealed for help in tracing movements of a green Jeep Cherokee in the days before it rammed an airport terminal in Glasgow on Saturday. The driver of the vehicle, who set himself on fire, remained in critical condition.

The attack came 36 hours after police found two Mercedes car bombs packed with fuel canisters, propane tanks and nails parked near a crowded nightclub in London’s teeming theatre district.

Police say the London and Scotland incidents are linked. On Sunday they raided a house in an affluent suburb about 10 minutes from the Glasgow airport, where neighbors said two Asian men had moved in just weeks ago. Most of the Asian population in Britain comes from the sub-continent, including India, Pakistan and Bangladesh.

Other arrests included a 26-year-old man and a 27-year-old woman seized on a major highway in northern England late Saturday, and a 26-year-old man in Liverpool on Sunday.

Authorities said fingerprints, cell phone records and surveillance video led to the arrests of a man and a woman on the M6 motorway near Cheshire. A security source, speaking on condition of anonymity, told NBC news the man is a doctor of Jordanian decent.

Staffordshire Police Chief Superintendent Steve Loxley said officers raided addresses in the central town of Newcastle-Under-Lyme near the highway where the two were arrested.

Fifth suspect arrested in LiverpoolMeanwhile, a search also was underway in Liverpool near the storied Penny Lane where a fifth suspect, a 26-year-old man, was arrested on Sunday. Two homes were being searched there, police said.

Sources tell NBC News the other suspects in custody include an Iraqi national and a Lebanese national, and that at least one suspect is still being sought.

Britain’s top-selling Sun newspaper identified one of those detained as an Iranian doctor who worked at North Staffordshire Hospital in central England. A spokeswoman at the hospital declined to comment on the case and police would not identify those detained.

Prime Minister Gordon Brown said "it is clear that we are dealing, in general terms, with people who are associated with al-Qaida." He warned Britons that the threat would be "long-term and sustained" but said the country would not be cowed by the plot targeting central London and Glasgow's airport.

"We will not yield, we will not be intimidated and we will not allow anyone to undermine our British way of life," he said in a nationally televised interview.

Frayed nervesThere were several scares in England on Sunday as authorities intensified their hunt for those behind the Glasgow and London incidents.

Late Sunday, a terminal at London’s Heathrow Airport was briefly closed after a “suspect package” was found, airport authorities said. The package was determined to be safe and the terminal was reopened.

Officers also carried out a controlled explosion on a car outside the Glasgow-area hospital where a suspect from the airport attack was being treated for severe burns, police said. No explosives were found.

‘Not born and bred here’
Also of concern amid this heightened state of alert: extremists living in the U.K. who may have received jihadist training in Iraq, and returned home with dangerous new skills.

Kenny MacAskill, Scotland’s justice secretary, said the two Glasgow attackers were not “born and bred here.”

“Any suggestion to be made that they are homegrown terrorists is not true,” he said.

NBC News has also learned that Scotland Yard five weeks ago began an intense manhunt for an Algerian-born man, and others, after they suddenly disappeared from a kind of home detention. British police had arrested the man and put him under restrictions after he allegedly discussed the bombing of London nightclubs.

On Sunday, NBC News obtained a "Threat Analysis" of the London car bombs, written by the New York Police Department, that says: "150 Britons have traveled to fight in Iraq; a number are believed to have returned and formed 'sleeper cells.’"

Britain on Saturday raised its terror alert to “critical” — the highest possible level — and the Bush administration announced plans to increase security at airports and on mass transit.

The new terror threat presents Brown with an enormous challenge early in his premiership — and comes at a time of already heightened vigilance one week before the anniversary of the July 7, 2005, London transit attacks. Those were largely carried out by local Muslims, exacerbating ethnic tensions in Britain.

Brown, a Scot who replaced Tony Blair as prime minister just days earlier, urged Britons to remain “constantly vigilant” about security. He said “Everything is being done in our power ... to protect people’s lives.”

'We will not be intimidated'
Lord Stevens, London’s former police chief, called it a major escalation in the campaign waged by Islamic militants.

In a column in Sunday’s News of the World newspaper, he wrote that the terrorists are using “the same technology, the same bomb-making techniques, the same operating methods as their brothers-in-arms in both Baghdad and Bali,” referring to the 2002 and 2005 attacks on the Indonesian resort island that killed more than 200 people.

Glasgow police chief Willie Rae said a suspicious device was found on a man wrestled to the ground by officers at Glasgow airport and hospitalized in critical condition with severe burns.

John Smeaton, who saw the attack, said the man shouted “Allah, Allah” as he was detained.

Glasgow airport began reopening Sunday, although the airport operator warned many flights would be canceled. The crashed Jeep remained out front, covered in a blue tarpaulin, and cars were not allowed to drive up to the terminal.

Memories of foiled 1999 plotOfficers also were reviewing closed-circuit television footage in the search for clues into the foiled London bombings.

The incident hinted of a foiled December 1999 plot to attack Los Angeles International Airport. Customs agents stopped a man in a car packed with 124 pounds of explosives. He is serving a 22-year prison sentence for the plot.

And last year, a 35-year-old British convert to Islam was convicted of plotting to bomb several U.S. financial targets and luxury London hotels with a plot that called for using limousines packed with gas tanks, napalm and nails.

In April, accused members of an al-Qaida-linked terror cell were convicted of conspiring to blow up the Ministry of Sound nightclub, one of London’s biggest music venues.

NBC News' Lisa Myers and Aram Roston, and The Associated Press and Reuters contributed to this report.